Polyhydroxyl compounds have acquired considerable commercial significance in a variety of different fields. They are commercially used, for example, for the production of non-ionic surface-active compounds, as anti-freeze agents, humectants, plasticizers and as starting components for plastics, such as polyester and polyether resins.
Polyhydric alcohols are currently obtained from naturally occurring substances, such as sugar or cellulose materials, or are synthesized by the oxidation of petroleum derivatives.
In view of the world food situation, however, it would appear to be poor policy to use naturally occurring materials (which may be used as a carbohydrate source in foods) as starting materials for commercial products. On the other hand, due to the shortage of petroleum resources, the prices of products dependent upon petroleum have steadily increased. In addition, there is no guarantee of the longterm availability of petroleum products. Accordingly, it is desirable to find production processes for polyhydroxyl compounds of which the raw material supply is dependent neither upon naturally occurring materials nor upon petroleum.
Since the works of Butlerow and Loew (ann. 120, 295 (1861) and J. pr. Chem. 33, 321 (1886)) it has been known that hydroxy aldehydes and hydroxy ketones are formed by the auto-condensation of formaldehyde hydrate (hereinafter the expression "auto-condensation of formaldehyde" is always intended to mean the "auto-condensation of formaldehyde hydrate") under the influence of basic compounds, such as calcium or lead hydroxide. Since formaldehyde may be obtained from coal or natural gas by way of methanol, this would, in principle, be one method of obtaining hydroxyl group-containing compounds from which polyhydric alcohols may be synthesized independent of petroleum. The method would be by electrolytic reduction or by catalytic and chemical hydrogenation.
However, despite numerous proposals for the synthesis of polyhydroxyl compounds by the auto-condensation of formaldehyde, no commercially workable process has as yet been developed for this purpose because it has not yet been possible to synthesize mixtures of polyhydroxyl compounds with defined reproducibility of the hydroxyl functionality. In addition, the hydroxy aldehyde and hydroxy ketone mixtures obtained in conventional processes require the use of large quantities of catalyst and are difficult to hydrogenate. This high consumption of catalyst has hitherto made the synthesis of polyhydroxyl compounds by the auto-condensation of formaldehyde hydrate appear uneconomic and has prevented the auto-condensation of formaldehyde hydrate from being used as the basis of a commercial process for the synthesis of polyhydric alcohols.
Due to the simultaneous disproportionation of the formaldehyde into methanol and formic acid, it has only been possible to obtain moderate yields by the conventional processes. Considerable costs are involved in working-up the aqueous or aqueous/alcoholic solutions formed.
The disproportionation of formaldehyde into methanol and formic acid is known to be catalyzed to a very considerable extent by basic compounds. As Pfeil, Chemische Berichte 84. 229 (1951) found, the reaction velocity of this so-called "Cannizzaro reaction" is dependent upon the square of the formaldehyde concentration, while the velocity of the formaldehyde polyaddition reaction (C-C-linkage) is a linear function of the formaldehyde concentration (Pfeil and Schroth, Chemische Berichte 85, 303 (1952)). Accordingly, with increasing aldehyde concentration, the desired quantitative ratio of polyhydroxyl compounds to methanol and formic acid is not obtained. Accordingly, in most conventional processes, condensation of the formaldehyde into hydroxy aldehydes and hydroxy ketones is carried out in solutions having low formaldehyde concentrations. However, in order to recover the hydroxy aldehydes and hydroxy ketones formed, the water used as solvent must be removed by distillation. This gives rise to considerable energy costs due to the considerable heat of evaporation of the water. For this reason, processes for condensing formaldehyde from dilute aqueous solutions are uneconomical. In addition, decomposition and discoloration reactions involving the hydroxy aldehydes and hydroxy ketones formed generally occur during the prolonged distillation times.
It is therefore desirable to carry out the condensation of formaldehyde from standard commercial-grade concentrated formalin solutions without troublesome secondary reactions. German Pat. No. 822,385 describes a process for the production of aliphatic oxyaldehydes, in which a 40% formalin solution is reacted with thallium or thallium hydroxide. However, this process is objectionable in view of the toxicity and availability of thallium. Moreover, the yields of this process, of from 70 to 80%, are relatively low.
In order to avoid the Cannizzaro reaction, it has also been proposed to react formaldehyde solutions with calcium or lead hydroxide in the presence of methanol, ethanol or other polar organic solvents (German Pat. No. 830,951 and Gorr and Wagner, Biochemische Zeitschrift, 262 261 (1933)). However, by adding organic solvents, the formaldehyde content of the solution is again reduced. The additional energy costs involved in evaporating the solvent added during working-up of the hydroxy aldehydes and ketones formed also make these processes uneconomical. In addition, substantially unstable semi-acetals are formed from formaldehyde and lower alcohols, decomposing during the condensation reaction.
German Pat. No. 884,794 describes a process for the production of oxy-oxo compounds, in which up to 30% aqueous formaldehyde solutions are reacted with lead oxide or lead acetate and inorganic bases to form sugar-like compounds which reduced Fehling's solution at cold temperatures. In this process, however, the formaldehyde solution has to be heated for from 7 to 8 hours. For this reason, the volume-time yield obtained is unsatisfactory. The relatively poor yields (about 80%, based on the formaldehyde used) are also unsatisfactory.
U.S. Pat. No. 2,224,910 describes a process for the production of hydroxy aldehydes and hydroxy ketones, in which the exothermic auto-condensation of the formaldehyde is controlled by the measured addition of inorganic or organic bases to a formaldehyde solution containing lead, tin, calcium, barium, magnesium, cerium or thorium compounds and a compound capable of enediol formation, such as glucose, ascorbic acid, fructose, benzoin, glycol aldehyde, erythrose, reductose, invert sugar or condensation products of formaldehyde. Although a mixture of hydroxy aldehydes and hydroxy ketones is obtained from formaldehyde solutions of relatively high concentration without the addition of organic solvents in this process, various disadvantages exist. When the reaction is carried out at low pH values, substantial quantities of hydroxy aldehyde and hydroxy ketone mixtures of low hydroxy functionality are obtained. In addition, only moderate reaction velocities are obtained at low pH values, so that the volume-time yields of the process are generally unsatisfactory. In order to obviate these disadvantages, it is recommended in the above-mentioned patent to start condensation of the formaldehyde at low pH values and then to complete the condensation at higher pH values. At pH values of .gtoreq.7, however, the lead-catalyzed auto-condensation of the formaldehyde takes place spontaneously and uncontrollably. It is, thus, not possible by this process to obtain mixtures of hydroxy aldehydes and hydroxy ketones with a reproducible component distribution. In addition, it is known that hydroxy aldehydes, hydroxy ketones and mono-saccharides decompose in alkaline medium and at elevated temperature to form dark colored compounds partially containing carboxyl groups.
These decomposition reactions occur in the processes suggested as preferred in U.S. Pat. No. 2,224,910, especially after most of the formaldehyde has reacted. Accordingly, hydroxy aldehyde and hydroxy ketone mixtures so produced contain decomposition products having acid groups, are brown in color and cannot be reproducibly obtained. In addition, these mixtures may only be hydrogenated using uneconomically large quantities of Raney nickel catalyst. For example, 30 g of Raney nickel are required for hydrogenating a quantity of hydroxy aldehyde and hydroxy ketone mixture equivalent to 100 g of formaldehyde.
For purification and for recovering hydroxyl compounds of low molecular weight, the product mixture obtained by the process just described always must be worked-up by distillation. This distillation necessarily involves additional energy and plant costs. It would be desirable to produce the product mixtures in such a way that they might be directly used after removal of the solvent water without any need for distillation. However colorless reaction mixtures substantially free from secondary products cannot be obtained by conventional processes.
Accordingly, an object of the present invention is to provide a process by which it is possible to synthesize mixtures of polyhydroxyl compounds which are substantially free from decomposition products and which may readily by hydrogenated using small quantities of hydrogenation catalysts to form polyhydric alcohols. The mixtures of polyhydroxyl compounds obtained should be colorless and should not require further purification.
Another object of the present invention is to control the auto-condensation of formaldehyde in such a way that the product distribution of the mixtures of low molecular weight polyhydroxyl compounds formed may be varied and reproducibly adjusted as required.
The term "formose" in the context of the present invention means the known mixtures of low molecular weight polyhydroxyl compounds (polyhydric alcohols, hydroxy aldehydes and hydroxy ketones) which are produced by the auto-condensation of formaldehyde hydrate.